In general, it is known that a radio wave is significantly attenuated in water with distance.
For this reason, there have been proposed various underwater communication systems.
Since a radio wave has a property of being attenuated in water, there have been proposed many communication systems that use an ultrasonic wave or light instead of a radio wave.
Since an ultrasonic wave or light is less significantly attenuated underwater than a radio wave, communication can be performed also underwater by a communication system and a communicating method that use an ultrasonic wave or light.
Alternatively, there have been proposed underwater communication systems that use a radio wave.
In the case where a radio wave is used for underwater communication, a wired telecommunication cable is typically arranged between communication devices to preclude the attenuation in water.
However, the wired telecommunication cable is inconvenient to handle, and to avoid the inconvenience, there has also been proposed an air passage formed between communication devices (Japanese Patent Laid-Open No 11-355219).
Alternatively, for the purpose to avoid the inconvenience of the wired telecommunication cable likewise, there has also been proposed a self-propelled relaying device provided between communication devices (Japanese Patent Laid-Open No. 2001-308766).
Now, there have been proposed various autonomous unmanned underwater robots that perform various operations underwater.
In general, since apparatuses such as batteries, control devices, and cameras tend to malfunction in nature when being exposed to water (non-water resistance), early type underwater robots have been constructed to contain all non-water-resistant apparatuses in a common water-resistant-pressure-resistant container (in a hull), and project an operation arm or a propulsion device from a required portion.
However, since a plurality of the propulsion devices or the like need to be spaced at a distance, the common water-resistant-pressure-resistant container that contains all the apparatuses has to be made large, which causes a problem of useless spaces arising in the container.
Hence, there have subsequently been developed and proposed modularized underwater robots.
In this modularized underwater robot, a single-function module such as an operation device, a power supply device, and a control device is constructed as a unit so as to be as independent as possible, and the control device manages and controls the whole operation of the underwater robot.
The structures of the independent units are configured such that single-function modules are contained in small-sized water-resistant-pressure-resistant cases and arranged in required positions and such that the water-resistant-pressure-resistant cases can communicate with one another through cables connected to connectors that are provided to the waterproof-pressure-resistant cases. The independent units are caused to be in functional corporation with one another to operate through communication among the single-function modules using the cables.